Interfacial thermodynamics of 2D-materials-based nanofluidic systems

Summary

Controlled aqueous ion flow through nanoporous membranes is a critical function of protein ion channels, which ubiquitously control salinity in living cells. Recent advances in fabrication make solid sub-nanometer pores in two-dimensional (2D) materials a reality. Such pores, often resulting from merely a dozen or so atomic sites ejected from the host 2D lattice, offer bio-like functionalities in artificially made systems and promise to revolutionize a diverse range of applied areas. To accurately predict the diverse functionality of these porous systems, an understanding of their thermophysical properties at the deep nanoscale level is of key importance. We leverage TRC’s core interdisciplinary expertise to conduct exploratory research at the cutting edge of the intersection of chemistry, thermodynamics, and mesoscale physics. We combine analytical theory, particle-based simulations, and quantum-chemical calculations to characterize the solid-liquid interfaces in and around sub-nm 2D pores and predict their charge and mass transport properties for applications in nanotechnology, including water desalination, biomolecule sensing, energy storage, and hybrid digital-analog computing. 

Description

Recent experiments have demonstrated that atomically symmetric sub-nanoscale pores can be fabricated in graphene, hexagonal boron nitride, molybdenum disulfide (MoS₂), as well as in other 2D materials. 

This development provides an intriguing bridge between nanofluidics and coordination chemistry, a seemingly unrelated field that describes the properties of entities such as crown ether molecules and molecular cages (Charles J. Pedersen, 1987 Nobel Prize in Chemistry).

The recently fabricated artificial pores, when immersed in water containing various solutes (e.g., metal ions or small molecules), fundamentally mimic the locally symmetric electrostatic environments featured by aqueous crown ethers, making the field of coordination chemistry directly apply to nanofluidic transport. In similarity to crown ethers, the solute-solvent and solute-pore interactions compete in a delicate manner, enabling these pores to sensitively “recognize” various solutes, depending on the effective size and electrostatic charge of the latter. Consequently, resulting from a combination of local energy barriers and entropic effects, the rich variety of transport properties of these sub-nm pores enables their promise for various applications.

We leverage TRC’s core competencies in coordination chemistry, physics, and materials science to theoretically and computationally investigate charge and mass transport through sub-nm pores in 2D membranes. First, we establish the relationships between pore structures (including geometry and composition) and their functionality by characterizing the local free energy surfaces for various solutes. We then estimate the corresponding transport properties and seek reliable ways to control them for specific applications.

A combination of analytical theory, particle-based simulations such as molecular dynamics (MD), and quantum-chemical calculations (e.g., density functional theory or DFT) is used to carry out our work using NIST’s computational resources. A large portion of our work is currently performed in collaboration with several research groups conducting experiments aimed at testing our theoretical predictions. Several other directions, including the fundamental and applied aspects of biopolymer transport through 2D nanopores, are being pursued as part of this effort.

Major Accomplishments

A new class of sub-nm pores with crown-like properties was demonstrated in graphene [1, 2] and MoS₂ [3].  For crown-like pores in graphene membranes locally suspended in aqueous salts, within tens of nanoseconds, K⁺ ions spontaneously organize and trap stably in the pores. The Na⁺ ions do not exhibit this behavior and the corresponding differences in local interactions are shown to cause significant differences between the transport behaviors of the two ion types [1]. We also demonstrated that a crown-porous graphene membrane in aqueous salt environment is an elementary logical NOT gate; two such membranes with independent electrostatic bias inputs constitute an ionic XOR gate [1].

The trapping strength for K⁺ was shown to be highly sensitive to slight membrane stretching, resulting in the first report of highly mechanosensitive ion channels [2]. Qualitatively similar phenomena were demonstrated for crown-like pores in molybdenum disulfide (MoS₂) [3]. In the case of ion transport from salt mixtures, we demonstrated ion sieving that is remarkably capable of (Na⁺/K⁺) permeation selectivity reversal, tuned by salt concentration and membrane strain [2]. This phenomenon potentially suggests promise for a number of applications, including ion sensing and a new generation of dialysis technology.

The studies aimed at experimental verification of these phenomena are currently underway in a recently formed collaboration with UC Berkeley, UC Merced, Lawrence Berkeley National Laboratory, and Lawrence Livermore National Laboratory.

Selected Publications

A. Smolyanitsky et al., ACS Nano, 2018. 12(7): p. 6677-6684.

A. Fang et al., Nat. Mater., 2019. 18(1): p. 76-81.

A. Fang et al., J. Phys. Chem. C, 2019. 123(6): p. 3588-3593.

A. Smolyanitsky and B. Luan, Phys. Rev. Lett., 2021. 127: p. 138103.